Every HID bulb has a maximum distance away it can support healthy plant growth and a minimum distance before light burn begins. The difference between these two figures is the maximum size plant you should grow under that bulb in a stationary setup. The smallest wattage bulb (or probably even better, one size up from smallest) that can accommodate the tallest strain you are growing all the way through harvest will often be the most efficient one to use.
Light movers spread out the light so higher wattage bulbs can be closer to the plants the faster they are moving. Once a light mover is brought into the equation, the efficiency of higher wattage lamps goes up considerably.
Only the top of the plant needs sufficient light to keep internodal length low so most HID's will create similar node spacing as long as the tops are kept within a few inches of the bulb's light burn zone.
This is all assuming that the lamp is properly cooled so that heat is not a problem in the healthy growth zone.
